A. Field of the Invention
Asphalt is used for a variety of purposes, including especially use in asphalt road paving systems and in roofing materials. The availability of asphalt materials has been reduced in recent years, while the cost of these materials has been increasing. For these and other reasons, there is great interest in finding ways to extend the useful life of asphalt containing products. In addition, asphalt road pavement and roofing materials may be exposed to a wide variety of weather conditions, including temperatures from below freezing to 100° F. or more. At colder temperatures, asphalt can become brittle and crack, while at higher temperatures, asphalt can permanently deform, for example by rutting in road pavements. Therefore, modifications that extend or improve the properties of asphalt in cold or hot conditions are also desirable.
Crumb rubber has been used is asphalt for decades to modify the properties of the asphalt. Crumb rubber may be obtained from grinding waste rubber products, such as for example rubber shoe soles. One of the most common sources of crumb rubber is ground tire rubber (GTR). It will be understood that references to GTR in the following description of the invention includes any source of rubber crumb that is suitable for addition to asphalt.
The mixing of GTR in asphalt or bitumen typically can lead to a heterogeneous blend with a liquid part and a solid material. The solid material settles when agitation of the GTR/asphalt mixture is stopped. The solid material is primarily carbon black, which has a significant negative impact on the workability of the GTR modified asphalt. The solid material mainly affects the viscosity and storage stability of the GTR modified asphalt. As a result of these drawbacks, the use of GTR in asphalt has been limited to some specific processes requiring special equipment. This can significantly increase the cost of pavement produced using the GTR modified asphalt.
B. Description of the Related Art
One prior process for producing GTR modified asphalt is described in International Patent Publication No. WO 95/20623. In the processes described in WO 95/20623, the GTR/asphalt mixture is heated and bombarded with air at a temperature of 350° and 470° F. The air is supplied at about 2200 cfm and 10 psi. The hot air dehydrogenates the GTR and homogenizes the GTR/asphalt mixture.
It would be desirable to have a process for producing GTR modified asphalt that allows use of conventional air blowing equipment in a more efficient way.